With the rapid development of the global economy, the shallow mineral resources of the earth are gradually depleted, and the development of resources continually goes deep into the earth. At the same time, the demands of human survival and development and the exploration on the unknown world are also constantly expanding the underground activity space. At present, many underground works that are being built and to be built in China continue to enter the deep part, for example, traffic tunnels, mine roadways, hydropower caverns, oil and gas storage caverns and the like are gradually developed to one or several kilometers. With the increase of excavation depths of underground works, the geological occurrence environment of deep cavern surrounding rock is increasingly complex. Under the “three-high and one-disturbance” condition of high geostress, high seepage pressure, high earth temperature and excavation disturbance, nonlinear deformation and failure phenomena significantly different from those in shallow buried caverns appear in deep caverns, e.g., large deformation, zonal disintegration, rock burst, water outburst and mud outburst, coal and gas outburst and other disasters often cause heavy casualties and economic loss and bad social impact. The traditional theories, methods and technologies of shallow rock mass have been unable to solve the nonlinear failure problem of deep rock mass. It is urgent to further study the nonlinear deformation characteristics and failure mechanism of the deep cavern surrounding rock. However, in the face of the complex nonlinear deformation and failure phenomena of the deep caverns, the traditional theories and methods are hardly competent, the numerical simulation is difficult, the in-situ test condition is limited and cost is very expensive, and by contrast, the geomechanical model test, which is vivid, intuitive and real, has become an important means of studying the nonlinear deformation and failure rules of the deep cavern. Different from MTS which can only study the mechanical characteristics of small-sized rock core specimens, the geomechanical model test is a physical simulation method for studying the engineering construction and deformation-failure process by using a reduced scale geological model according to the similarity principle. The geomechanical model test can provide supplementation and validation for the numerical simulation, can subtly simulate the nonlinear deformation and failure process of underground cavern excavation and the global degree of safety of a cavern group system, and plays an irreplaceable important role in discovering new phenomena, revealing new mechanisms, exploring new rules and validating new theories. In order to carry out the model test of underground engineering, it is necessary to have a geomechanics model test system. At present, the research status of the related geomechanical model test system is as follows:
(1) International Journal of Rock Mechanics and Mining Sciences, Issue 44, 2007, introduced a three-dimensional model test system for simulating mining, which cannot implement true three-dimensional non-uniform loading/unloading.
(2) Tunnelling and Underground Space Technology, Issue 23, 2008, introduced a geomechanical model test system for simulating sandy soil pipe roof construction, and this system can only implement geostatic stress loading, but cannot realize true three-dimensional non-uniform loading/unloading.
(3) International Journal of Engineering Geology, Issue 121, 2011, introduced a PFESA (Physically Finite Elemental Slab Assemblage) model test system, and this system can only implement uniform loading of plane stress, but cannot realize true three-dimensional non-uniform loading/unloading.
(4) International Journal of Rock Mechanics & Mining Sciences, Issue 48, 2011, introduced a quasi-three-dimensional geomechanical model test system, the single-sided maximum working pressure of the system is 300kN, and the system can simulate the state of plane stress, but cannot realize true three-dimensional non-uniform loading/unloading.
(5) Tunnelling and Underground Space Technology, Issue 50, 2015, introduced a plane stress test device, and the device can only implement vertical self-weight loading, but cannot realize true three-dimensional non-uniform loading/unloading.
(6) International Journal of Engineering Geology, Issue 197, 2015, introduced a model test system, the system applies air pressure via a built-in airbag in a model, the circumference of the model is passively constrained, and true three-dimensional non-uniform loading/unloading cannot be realized.
(7) Journal of Wuhan Hydraulic Power University, Issue 5, 1992, introduced a plane stress loading system, in which pressure is controlled by an air pump to be gradually loaded or unloaded, the system can only carry out plane loading, but cannot realize true three-dimensional non-uniform loading/unloading.
(8) Chinese Journal of Rock Mechanics and Engineering, Issue 3, 2004, introduced a multifunctional geotechnical engineering simulation test device, composed of upper and lower cover plates, a triangular distribution block and three sets of mutually perpendicular orthogonal pulling rod systems, and the device loads small-sized specimens and cannot realize high geostress non-uniform loading/unloading.
(9) Journal of Hydraulic Engineering, Issue 5 , 2002, introduced a discrete multi principal stress surface loading test system, the loading system is mainly composed of a high-pressure airbag, a reverse thrust plate, a limiting jack and an air compressor, and the loading system cannot realize high geostress true three-dimensional non-uniform loading/unloading.
(10) China Civil Engineering Journal, Issue 12, 2005, introduced a rock-soil geomechanical model test system, mainly composed of a bench counterforce device, a variable load loading plate and a hydraulic loading control test bed, and the system can only load plane strain with a limited loading value, but cannot implement true three-dimensional loading/unloading of a deep cavern.
In general, the existing geomechanical model test systems at home and abroad have the following problems:
1) The loading counterforce device of the model test system is fixed in size, and cannot be randomly adjusted according to the range of the test model;
2) The model test system is mainly based on planar, quasi three-dimensional, small-sized and uniform loading, and cannot implement the true three-dimensional non-uniform loading/unloading process;
3) The model test system loads a small load, and cannot truly simulate the high stress distribution state of deep rock mass via ultrahigh pressure loading;
4) The model test system cannot automatically acquire the displacement of any part inside the model.